1. Field of the Invention
The present invention relates to an apparatus for driving a light emitting element that can be applied to a light source or a backlight unit, and more particularly, to an apparatus for driving a light emitting element that can limit heat generation of a constant-current circuit including a metal oxide semiconductor (MOS) transistor by limiting a voltage applied to the constant-current circuit required for supplying a constant current to the light emitting element.
2. Description of the Related Art
In general, a light emitting element is an element emitting light. Examples of the light emitting element include a light emitting diode (LED), a laser diode (LD), and an organic light emitting diode (OLED).
The LED, which is one of the light emitting elements, is applied to various fields such as a lighting unit and a backlight unit, and will be applied to various fields in the future.
Two methods are used in driving the LED. One is using DC/DC of a switching mode, and the other is using a current source. Since the method of using the current source has not only a small switching noise but also a simple circuit, it is widely used. However, a heat generation from a MOS transistor included in the current source should be solved.
Hereinafter, a related art apparatus for driving an LED using a current source is described.
FIG. 1 is a view illustrating the construction of a related art apparatus for driving an LED. The apparatus for driving the LED includes a power unit 10 supplying driving power V required for driving a plurality of LEDs, which are light emitting elements, an LED unit 20 including the plurality of LEDs connected to the power unit 10, lit by the driving power from the power unit 10, and connected to each other in series, and a constant-current circuit unit 30 connected between the LED unit 20 and a ground to maintain a constant current flowing through the LED unit 20.
The constant-current circuit unit 30 includes a MOS transistor MOS including a drain connected to the cathode of the plurality of serially connected LEDs of the LED unit 20, a gate and a source, a sensing resistor RS connected between the source of the MOS transistor MOS and the ground, and a comparator 31 comparing a detection voltage VD detected by the sensing resistor RS with a predetermined reference voltage Vref to supply a tuning voltage VT determined by a difference between the two voltages to the gate of the MOS transistor MOS.
In the related art apparatus for driving the LED of FIG. 1 having the above construction, a current flowing through the LED unit 20 can be maintained constant using the constant-current circuit unit 30 supplying a constant current to the LED unit 20.
At this point, a current ILED flowing through the LED unit 20 is determined by the reference voltage Vref of the comparator 31 and the sensing resistor RS between the MOS transistor MOS and the ground as expressed by Equation 1.
                    ILED        =                              Vref            RS                    .                                    Equation        ⁢                                  ⁢        1            
However, in the related art apparatus for driving the LED of FIG. 1, as the driving voltage Vcc increases, a drain-source voltage Vds of the MOS transistor MOS increases. When the drain-source voltage Vds increases, heat is generated from the MOS transistor MOS.
Also, in the case where the LED included in the LED unit 20 is a high power LED, a current flowing through the LED unit 20 increases even more and thus heat generation becomes serious.